The wet process manufacture of phosphoric acid has been used for many years. The wet process involves the reaction of phosphatic solid materials, hereinafter termed phosphate rock, wherein calcium sulfate, monocalcium phosphate, phosphoric acid and sulfuric acid comprise the usual reaction media. The names of the three processes for the production of phosphoric acid by the wet process are based on the by-product calcium sulfate produced; namely, the gypsum or dihydrate process, the hemihydrate process, and the anhydrite process. Wherein the type of the by-product is dependent upon a number of factors, most importantly, the temperature of the process system and the P.sub.2 O.sub.5 concentration of the liquid phase of the slurry. For better control of that reaction temperature, P.sub.2 O.sub.5 concentration in the slurry, and higher P.sub.2 O.sub.5 yields, it has been a common practice to predry the phosphate rock before grinding to the proper reaction size. Any moisture introduced with the phosphate rock reduces the filtrate wash water and lowers P.sub.2 O.sub.5 yields thereby since evaporation of water is limited by the associated heat of reaction and dilution.
The BPL contents of commercially available phosphate rock ranges between 60% to 80%, with better grades of rock being rapidly depleted. Wherefore, high energy costs for drying and more rigid pollution effluent requirements have and are causing the phosphate industry to change to a practice of wet grinding the phosphate rock. The present method of wet phosphate rock grinding results in a phosphate slurry of about 65% solids and 35% water. With this increased water quantity to the phosphoric acid plant, the water balance problem even for present low filtrate phosphoric acid concentrations of 26-27% P.sub.2 O.sub.5 becomes extremely more difficult. Plants, employing wet phosphate rock grinding, have had to lower the wash water to the filter in an effort to increase the concentration of filtrate phosphoric acid. But this decrease in wash water input results in lower P.sub.2 O.sub.5 yields as the water balance within the process becomes critical.
It is therefore an object of the present invention to provide an improved method of wet process phosphoric acid production wherein increased inputs of water, either from undried phosphate rock, wet grinding of phosphate rock, use of dilute sulfuric acid, or from other sources will not interfere with associated high P.sub.2 O.sub.5 yields, lower concentrations of filtrate phosphoric acid and/or unbalance the zero steam requirements.
It is a further object of the present invention to allow for the use of increased inputs of water, either from undried phosphate rock, wet grinding of phosphate rock, use of dilute sulfuric acid, or from other sources whereby increased yields of concentrated phosphoric acid are achieved without changing the wet process reaction plant or procedure so as to be readily incorporated into existing production facilities.
It is a still further object of the present invention to allow for the use of increased inputs of water either from undried phosphate rock, wet grinding of phosphate rock, use of dilute sulfuric acid, or from other sources whereby increased yields of concentrated phosphoric acid are achieved and a substantial reduction in electrical and heat energy consumption is realized.